Polytopic multiplex holography

ABSTRACT

Disclosed is a multiplexing method and apparatus that allows holograms to be spatially multiplexed with partial spatial overlap between neighboring stacks of holograms. Each individual stack can additionally take full advantage of an alternate multiplexing scheme such as angle, wavelength, phase code, peristrophic, or fractal multiplexing, for example. An amount equal to the beam waist of the signal beam writing a hologram separates individual stacks of holograms. Upon reconstruction, a hologram and its neighbors will all be readout simultaneously. An filter is placed at the beam waist of the reconstructed data such that the neighbors that are read out are not transmitted to the camera plane. Alternatively, these unwanted reconstructions can be filtered out with an angular filter at an intermediate plane in the optical system that has a limited angular passband.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. ProvisionalPatent Application No. 60/453,529, filed Mar. 10, 2003 entitled “AMethod for Overlapping Holograms Using Location Based Filtering toSeparate Out the Signal” which is incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

[0002] Multiplexing holograms means to store multiple holograms in thesame volume or nearly the same volume. Typically, this is done byvarying an angle, wavelength, phase code, or some other system parameterin the recording and readout setup. Many of these methods rely on aholographic phenomena known as the Bragg effect in order to separate theholograms even though they are physically located within the same volumeof media. Other multiplexing methods such as shift and, to some extent,correlation use the Bragg effect and relative motion of the media andinput laser beams to overlap multiple holograms in the same volume ofthe media.

[0003] Some multiplexing techniques use momentum (spatial frequency) tofilter out the unwanted reconstructions from the desired reconstruction.Examples of these methods include: fractal, aperture (disclosed, forexample, in U.S. Pat. No. 5,892,601 to Curtis et al for “MultiplexHolography, which is incorporated in its entirety by reference) andperistrophic multiplexing. Each of which is understood in the art. Fordisclosure of aperture multiplexing see U.S. Pat. No. 5,892,601 which isincorporated in its entirety by reference, and for a disclosure ofperistrophic multiplexing see K. Curtis et al “Method of HolographicStorage Using Peristrophic Multiplexing”, Optics Letters, Vol. 19, No.13 pp. 993-994, 1994 and U.S. Pat. No. 5,483,365 to Pu et al. for“Method for holographic Storage using Persitrophic Multiplexing” each ofwhich is incorporated in its entirety by reference. By changing thereference beam angle and moving the media between recordings, thereconstructions are still Bragg matched but come out at different anglesand can therefore be filtered out.

[0004] Using holography to store data has been well known for the last30 years. The idea of increasing system capacity by combining spatialmultiplexing (recording holograms in multiple locations but notsignificantly in the same volume of media) along with some othermultiplexing technique that overlaps holograms within the same locationhas been well known for over 15 years. These are standard techniques fordistributing holograms on holographic media such as a disk, card, cube,or tape. Several patents and papers disclose a number of multiplexingtechniques: U.S. Pat. No. 5,550,779, “Holographic Memory with Angle,Spatial and Out of Plane Multiplexing”, and S. Li, “Photorefractive 3-DDisks for Optical Storage and Artificial Neural Networks” CaliforniaInstitute of Technology, pp. 78-111, 1994, each of which is herebyincorporated by reference. All of these place the beam waist, that is,the point at which the beam is focused and the beam spot size issmallest, (either image or Fourier transform plane) inside the media. Bydoing so, relatively small holograms can be generated which makeexcellent use of the media material's dynamic range.

[0005]FIG. 1 illustrates a prior art method of spatial and anglemultiplexing holograms in a relatively thick media. FIG. 1 shows aholographic media 8 in which an angle multiplexed hologram is beingcreated by reference beam 20 a and signal beam 10 a. In FIG. 1, signalbeam 10 a includes an incoming converging cone 12, an outgoing divergingcone 14 and a waist 16, where the signal beam is focused in the media 8and where its spot size is smallest. FIG. 1 also shows reference beam 20b, which can be used to generated a second hologram in media 8 that isangle multiplexed with the hologram generated by reference beam 20 a andsignal beam 10 a. A number of holograms, or stack, can be anglemultiplexed in a portion 24 of the media. The media or signal source canthe be shifted to record a second stack of holograms. FIG. 1 illustratessignal beams 10 b, 10 c and 10 d which, along with reference beams shownin phantom, generate additional stacks of holograms in portions 24 b, 24c, and 24 d, respectively, of media 8. In FIG. 1, the portions 24 a-24 dof media 8 outline the area used by each stack.

[0006] Portions 24 a-24 d are significantly larger than an individualbeam waist, such as beam waist 16. This is because both the signal beamand the reference beams determine the area that a given hologram stackuses. To spatially multiplex these holograms, stacks of holograms mustbe separated by at least the length of a portion 24 a-24 d of media 8.This has consequences for achievable densities and capacities that canbe reached using holographic storage. High density is achieved bymultiplexing more holograms in one location and by placing these stacksas close as possible. However, as discussed above, close spacing ofstacks is limited.

[0007] Additionally, the divergence of a beam can limit the minimumdistance between stacks. The amount of divergence, which can beexpressed as the angle the edges of the diverging cone form with thedirection of beam propagation, is dependent on the numerical aperture ofa lens through which the signal beam is projected. For high NA systemsthat are typically used for storage systems, the amount of signal beamdivergence in holographic media, such as media 8, is relativelysignificant for relatively thick media. In addition, the number ofholograms that can be multiplexed at one place (one stack) is determinedby the thickness of the media. More holograms can be stored in thickermedia due to the increase in the Bragg selectivity and dynamic range.Unfortunately, if the media is made thicker the spatial stack sizeincreases due to the increased divergence of the beam. Thus theachievable density/capacity saturates at a certain thickness. Thus,density cannot be increased significantly by increasing the materialthickness once the saturation thickness is reached.

[0008] Increasing density is also possible by overlapping holograms. Anexample of this with angle multiplexing is disclosed in “SpatioangularMultiplexed Storage of 750 Holograms in an FeLiNbO3 Crystal”, OpticsLetters, Vol. 18, No. 11 pp. 912-914, 1993, which is incorporated in itsentirety by reference. With this concept, partially overlapping hologramstacks are recorded with angle multiplexing within each stack. Eachstack, however, has a unique set of angles and therefore, though thestacks partially overlap, the holograms can be easily separated. Thisincreases the density of the stacks but many fewer holograms can berecorded in a stack, which very significantly reduces the density gainsof overlapping the stacks. In practice this method results is verylittle if any increase in achievable density. When multiplexingholograms, however, the dynamic range of the holographic media can be alimiting factor. (The materials dynamic range or M# is a measure howmany holograms can be multiplexed at a given location in the materialand is related to the materials index change and material thickness.)Thus, the reduced possible number of angle multiplexed holograms wasacceptable since it reduced the demands on the available dynamic rangefor a given overall density. This is because as more holograms aremultiplexed in the same volume (i.e. angle multiplexed) the diffractionefficiency of the holograms drops depending the material dynamic range(M#) divided by the number multiplexed holograms squared. Now thatbetter materials have been invented, a way of actually increasing theachievable density is needed.

BRIEF SUMMARY OF THE INVENTION

[0009] This invention describes a new holographic recording techniquereferred to herein as Polytopic multiplexing. This multiplexingtechnique allows holograms to be spatially multiplexed with partialspatial overlap between neighboring stacks of holograms. Each individualstack can additionally take full advantage of an alternate multiplexingscheme such as angle, wavelength, phase code, peristrophic, correlation,or fractal multiplexing. An amount equal to the beam waist of the databeam writing the hologram separates the individual stacks of holograms.Upon reconstruction, the data and its neighbors will all be readoutsimultaneously, however, an aperture (filter) is placed at the beamwaist of the reconstructed data such that the neighbors that are readout don't make it to the camera plane and are thereby filtered out.Alternatively, these unwanted reconstructions can be filtered out withan angular filter having a limited angular passband.

[0010] In particular, in a method for forming and reproducing a hologramin accordance with the present invention a first hologram creating afirst hologram in a holographic media using a first reference beam and afirst signal beam, the first signal beam having a waist. A secondhologram is created using a second reference beam that is the same asthe first reference beam and a second signal beam, the second signalbeam also has a waist. At least a portion of the second hologram isspatially overlapped with the first hologram. However, the firsthologram is spatially separated from the second hologram such that noportion of the waist of the first signal beam occurs in the samelocation as any portion of the waist of the second signal beam. Thefirst hologram is regenerated in a first portion of an output beam andat least the second hologram is regenerated in a second portion of theoutput beam. The output beam is filtered to substantially contain only areadout of the first hologram.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a diagram illustrating prior art multiplexing ofmultiplexed stacks of holograms in holographic media.

[0012]FIG. 2 is a diagram illustrating overlapping a plurality ofmultiplexed holograms in holographic media in accordance with thepresent invention.

[0013]FIG. 3 illustrates a system and method for overlapping a pluralityof multiplexed holograms using a signal beam having a beam waist outsidethe holographic media and reading out individual holograms in accordancewith the present invention.

[0014]FIG. 3a illustrates a system and method for overlapping aplurality of multiplexed holograms using a signal beam having a beamwaist inside the holographic media and relaying the beam waist to alocation outside the holographic media in accordance with the presentinvention.

[0015]FIG. 4 is a diagram of a filter block for filtering out unwantedholographic readouts for use in a method and system in accordance withthe present invention.

[0016]FIG. 4a is a diagram of holographic media including a filter forfiltering out unwanted holographic readouts for use in a method andsystem in accordance with the present invention.

[0017]FIG. 5 illustrates a system and method for overlapping a pluralityof multiplexed holograms using a signal beam having a beam waist abovethe holographic media in accordance with the present invention.

[0018]FIG. 6 illustrates phase conjugate readout of a hologram generatedby the system and method shown in FIG. 5.

[0019]FIG. 7 illustrates a system and method for overlapping a pluralityof holograms in a holographic media and including no lens between aspatial light modulator and the holographic media in accordance with thepresent invention.

[0020]FIG. 8 illustrates lens-less, phase conjugate readout of ahologram generated by the system and method shown in FIG. 7.

[0021]FIG. 9 illustrates a system and method for overlapping a pluralityof holograms in a holographic media and including a relay lens systemfor relaying the waist of a signal beam to a location outside theholographic media in accordance with the present invention.

[0022]FIG. 10 illustrates phase conjugate readout of a hologramgenerated by the system and method shown in FIG. 9.

[0023]FIG. 11 illustrates a system and method for overlapping aplurality of holograms in a holographic media using two angular filtersin accordance with the present invention.

[0024]FIG. 12 illustrates a system and method for generating areproduction of the holograms generated by a system and method inaccordance with the present invention.

[0025]FIG. 13 illustrates a system and method for overlapping aplurality of holograms in a circular holographic media in accordancewith the present invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0026] A method of increasing the density of holograms stored in aholographic media in accordance with the present invention isillustrated in FIG. 2. FIG. 2 illustrates a holographic media 108 inwhich a plurality a holograms are generated by a plurality of signalbeams 110. Each of the plurality of signal beams 110 include an incomingconverging cone, and outgoing diverging cone and a beam waist. Inparticular, a first signal beam 110 a includes a first incomingconverging cone 112 a, and a first outgoing diverging cone 114 a and afirst beam waist 116 a. As used herein, the “waist” of a beam canindicate either the beams Fourier transform plane or image plane. InFIG. 2, beam waist 116 a can be either the Fourier transform plane orthe image plane. FIG. 2 also shows a first reference beam 120 a and asecond reference beam 120 b. As is understood in the art, signal beam110 a and first reference beam 120 a can generate a first hologram. Afirst additional signal beam (not shown) which can be spatiallycoincident with first signal beam 110 a and contain differentinformation that first signal beam 110 a, can generate, as is alsounderstood in the art, a first additional hologram that is angle, orotherwise, multiplexed with the first hologram at a first stack locationin media 108 which is spatially coincident with converging cone 112 a.As understood in the art, further additional holograms can be anglemultiplexed together at this first stack location. It is considered thatholograms multiplexed together in a single stack can be multiplexed byany method other than angle multiplexing including wavelength,peristrophic, correlation, or phase code multiplexing, for example.

[0027] A second signal beam 110 b, which is not spatially coincidentwith first signal beam 110 a, can generate a second hologram at a secondstack location which is spatially coincident with a second convergingcone 112 b of second signal beam in media 108. Additional secondholograms can be angle or otherwise multiplexed with the second hologramat the second stack location using additional second signal beams thatare spatially coincident with, but carry different information from,second signal beam 110 b.

[0028] Second signal beam 110 b includes a second incoming convergencecone 112 b, a second outgoing diverging cone 114 b and a second beamwaist 116 b. As shown in FIG. 2, second signal beam 110 b is directedsuch that second converging cone 112 b partially spatially overlaps withfirst converging cone 112 a of first signal beam 110 a inside of media108. As such, when reproducing the first hologram in an output beam,information from the second hologram (and potentially other hologramscreated by other signal beams 110) will be included in the output beamafter the readout beam passes through media 108. Therefore, and asdiscussed in detail below, a filter block 130 adjacent to media 108 andin the path of a output beam, is used to filter out information from asecond, and potentially other, holograms which will also be included inthe output beam.

[0029] Such filtering of an output beam is possible because while secondsignal beam 110 b is preferably directed such that second convergingcone 112 b partially spatially overlaps with first converging cone 112a, second signal beam 110 b is also preferably directed such that secondbeam waist 116 b does not spatially overlap with first beam waist 116 a.Thus, filter block 130 is preferably placed at the location of the waistof the signal beam that generated the hologram that is desired to bereproduced. Filter block 130 is designed to allow a portion of an outputbeam containing information from only the first hologram and also havinga waist at this location to pass substantially un-attenuated, whileblocking readouts of holograms generated by signal beams that overlappedthe signal beam that generated the desired hologram.

[0030]FIG. 3 is a diagram illustrating a holographic system 200 forcarrying out a method in accordance with the present invention.Holographic system 200 is for generating and reading out holograms inholographic media 208. Holographic system 200 includes a reflectivespatial light modulator (SLM) 235 for providing data for holograms to berecorded in media 208. SLMs are well understood by those skilled in theart. Adjacent to SLM 235 is a beam splitter 240 which directs anincident signal beam 250 off of partially reflecting mirror 240 a, ontoSLM 235 and through first Fourier transform (FT) lens 242, which, in theembodiment of FIG. 3, consists of 2 elements. Holographic system 200also includes a second FT lens 244 for focusing a regenerated hologramonto a detector 246. It is also considered that FT lens 244 be aquasi-FT lens. Media 208 can be any media capable of storing holograms,but preferred media includes media available under the Tapestry™ brandname from Inphase technologies of Longmont, Colo. Such media includes aphotopolymer disclosed in U.S. Pat. No. 6,482,551 to Dahr et al. for“Optical Article and Process for Forming Article” which is incorporatedby reference in its entirety. Also, media 208 can be in the form of acard having rectangular or other shape or in the form of a tape.

[0031] FT lens 242 directs incident signal beams through media 208 togenerate, along with a reference beam 220, a plurality of hologramstherein. As discussed above, a plurality of holograms may be multiplexedin known manners at a single location in media 208. After generating atleast a first hologram at a first location in media 208, represented inFIG. 3 by a converging cone 212 a, media 208 and the combination of SLM235, beam splitter 240 and ST lens 242 can be shifted with respect toeach other in a known manner to generate additional holograms,represented in FIG. 3 by converging cones 212 b and 212 c, which are notat the first location in media 208. As shown in FIG. 3, holograms atconverging cones 212 a, 212 b and 212 c preferably overlap with eachother. As such, groups or stacks of multiplexed holograms are recordedin a line in media 208. As also discussed above, the waists of theincident beams generating the holograms at converging cones 212 a, 212 band 212 c however, do not spatially overlap.

[0032] It is to be understood that holograms generated at convergingcones 212 b and 212 c may be generated using the same reference beamused to generate a hologram at converging cone 212 a. As used herein,“same” reference beam indicates a reference beam having substantiallythe same characteristics such as angle of incidence, phase, andwavelength, for example, as a comparison reference beam but that mayotherwise be shifted in space or time. As such the same reference beamcan generate a hologram with two different signal beams at differenttimes and at different locations. Thus, if there are a plurality ofholograms angle multiplexed at converging cone 212 a, hologramsgenerated at converging cones 212 b and 212 c may be generated usingreference beams having substantially the same multiplexing angles,phases, wavelengths, wavefronts, etc., as those generated at convergingcone 212 a even though the holograms at converging cones 212 b and 212 cwill overlap with holograms at converging cone 212 a of the samemultiplexing angle, phase, wavelength, wavefront, etc.

[0033]FIG. 3 illustrates readout of a hologram generated in media 208 atconverging cone 212 a. A readout beam which is the same as referencebeam 220 and is spatially coincident with reference beam 220 can be usedto regenerate the hologram at converging cone 208. However, as discussedabove, because holograms have also been created at converging cones 212b and 212 c using the same reference beam that created the hologram atconverging cone 212 a, and which overlap with the hologram at convergingcone 212 a, the holograms at converging cones 212 b and 212 c will alsobe regenerated with a readout beam that is the same as reference beam220 and spatially coincident therewith.

[0034] In order to avoid the holograms at converging cones 212 b and 212c from be detected by detector 246, a filter block 230 is used to filterout the readouts generated from holograms at converging cones 212 b and212 c. As noted above, signal beams generating holograms at convergingcones 212 a, 212 b and 212 c do not overlap at the beam waists. And, asshown in FIG. 3, the beam waists are positioned outside media 208. Assuch, filter block 230 can be positioned to allow a portion 215 of anoutput beam 211 carrying a readout of the hologram at converging cone212 a to pass through second FT lens 244 and into detector 246 whileblocking portions of the output beam carrying readouts of the hologramsat converging cones 212 b and 212 c.

[0035]FIG. 4 is a diagram illustrating a filter block 230 which may beused to carryout such filtering. Filter block 230 preferably includes anopaque block 232 having a hollowed out area 234 in the form of a 4-sidedtruncated pyramid. Area 234 includes a first square aperture 236 in anupper face of opaque block 232 and a second square aperture 238 in alower face of opaque block 232. Preferably, in use, filter block 230 ispositioned such that upper aperture 234 is nearest to media 208 and issubstantially perpendicular to the direction of propagation of outputbeam portion 215. To reproduce the hologram stored at converging cone212 a, filter block 230 is also preferably positioned such that thewaist of a portion 211 of an output beam carrying a readout of thehologram at converging cone 212 a can substantially pass throughaperture 234 and the waists of the portions of the output beam that arereproducing holograms at converging cones 212 b and 212 c are blocked byfilter block 230. Any filter can be made that limits the size of thebeam waist. If the filter is in a relay system, as described below, theconfiguration of the block is less critical because the filter block canbe placed further away from the media thus reducing physicalinterference with the reference beam and the media.

[0036] In the example illustrated in FIG. 3, the 2 FT lenses are in whatis commonly called a 4F imaging system arrangement. System 200 couldalso be relayed by an additional 4F system between detector 246 and FTlens 244. Filter 230 can then be placed in the Fourier plane of this 4Fsystem. This allows for the beam waist to be placed inside the media andstill achieve filtering of unwanted hologram readouts and stack overlap.Such a system 200′ is shown in FIG. 3a. System 200′ includes an SLM235′, beam splitter 240′ first FT lens 242′, media 208′ second FT lens244′ and detector 246′, each as described above with respect to system200. System 200′ also includes an additional 4F lens system includingthird FT lens 260′ and fourth FT lens 262′ between second FT lens 244′and detector 246′. Additionally, a filter block is not placed betweensecond FT lens 244′ and media 208′. Rather, a filter block 230′ ispositioned between third FT lens 260′ and fourth FT lens 262′. FT lenses242′, 244′, 260′ and 262′ can also be quasi-FT lenses.

[0037] System 200′ records multiplexed and overlapped holograms in media208′ as described above with respect to system 200. FIG. 3a illustratesreadout of a hologram from media 208′. A readout beam 220′, which is thesame as a reference beam that was used to generate a hologram desired tobe read, is used to generate an output beam 211′ that includes a portion215′ which contains a readout of the desired hologram. After passing outof media 208′ the output beam passes through second FT lens 244′ andthird FT lens 260′, which focuses the output beam portion 215′ to asecond beam waist 216 b before reaching fourth FT lens 262′. In system200′, holographic media 208′ is preferably located at the Fouriertransform place of the object beam of system 200′.

[0038] As discussed above, a reference beam that is the same as readoutbeam 220′ was used to create holograms overlapping with the hologramreadout in output beam portion 215′. Thus, readouts (not shown) of theseadditional holograms will also be included in the output beam 211generated by readout beam 220′. Because, as discussed above, media 208′was shifted by at least a distance equal to the diameter of a beam waist216 b of signal beam 210, beam waists of portions of output beam 211containing readouts of additional holograms will not overlap with secondbeam waist 216 b of portion 215′ of output beam 211. Thus, filter block230′ is preferably positioned to block transmission of portions ofoutput beam 511 other than portion 215′ at second beam waist 216 bthereof. In this way, only portion 215′ of output beam 211 istransmitted to fourth FT lens 262′ and detector 246′. If there ismagnification in system 200 enabled by lenses 244 and 260 then thedistance to move the media is the magnified distance of the beam waist216 b.

[0039] By using a second 4F lens system including FT lenses 260′ and262′ to generate a second beam waist 216 b′ of output beam portion 215′outside media 208′, as shown in FIG. 3a, first beam waist 216 a′ can beplaced inside media 208′. Having the beam waist inside the media has theadvantage of making the best use of the material dynamic range. Inaddition, and as discussed below with respect to FIGS. 9 and 10, a 4Frelay system could be placed between beam splitter 240′ and the first FTlens 242′. A filter block like filter block 230′ can then be placed atthe beam waist generated before first FT lens 242′ thus limiting thesignal bandwidth and decreasing the size of generated holograms. Thisdecreases the size of a stack in the holographic media and reducedhigher order reflections of SLM 235′. A transmissive SLM can also beused and are well known in the art. Other lens arrangements to relay orimage the aperture or the SLM image are also possible.

[0040] Additionally, aperture 234 must be sized to allow enoughinformation to reproduce the hologram at converging cone 212 a to pass.To accomplish this, the length of the sides of aperture 234 can be givenby:

L=(γ)(focal length)/pixel diameter

[0041] where “L” is the length of the sides of aperture 234, “γ” is thewavelength of output beam portion 215, “focal length” is the focallength of FT lens 242 used to generate the hologram, and “pixeldiameter” is the diameter of a single pixel of SLM 235. The L calculatedabove, referred to as the Nyquist size or Nyquist aperture, is largeenough to pass the information of the pixels but limit error rates. TheNyquist size L is the preferred size for aperture 234 though other sizesthat are larger or smaller than the Nyquist size may also be used. Forexample, and without limitation, an aperture dimension of either ½L or2L may also be used. Though filter block 230 includes square apertures234 and 238, apertures in a filter block that may be used in accordancewith the present invention may have an aperture of any shape. It is alsocontemplated to use aperture sizes not given by the above equation andwhich may yield more or less information about the hologram beingreproduced. The smaller the aperture or passband of the filter, the moredensity gain is realized until the signal to noise ratio drops below therecoverable limit with reasonable error correction and filtering of thesignal. Preferably, an average diameter of beam waist such as beam waist216 a, and therefore, the average side of an aperture 234, is on theorder of 0.5 mm to 2 mm, but may be either larger or smaller.

[0042] As discussed above, the filter can be in the optical system butit is possible that the filter could be made part of the media. FIG. 4bis a diagram illustrating one example of a holographic media 270including a filter for use in a method and apparatus of the presentinvention. Holographic media 270 is a rectangular strip of holographicmedia as understood in the art. However, media 270 includes an opaquetop surface 274 having a plurality of square apertures 272. Media 270 iscontinuous within apertures 272, however opaque top surface 274 isinterrupted within apertures 272 such that a beam projected onto opaquetop surface 274 of media 270 may pass into media 270 through apertures272. Thus, media 270 could be used, for example, in place of media 208and filter block 230 in system 200. To maximize the density of hologramsin system 200 using media 270, apertures 272 would be located on theside of the media closest to FT lens 244 and beam waists would belocated at the face of media 270 containing apertures 272. Multipleapertures in media 274 allow stacks of holograms to be multiplexed atmultiple locations in media 270.

[0043] It is also considered that an angular filter, discussed in detailbelow, be used in a method an apparatus of the present invention inplace of a filter block, such as filter block 230, or media 270.

[0044] Preferably, when media 208 or 208′ is shifted to recordadditional holograms therein, it is preferably shifted by a distancesubstantially equal to the diameter of beam waist 210. In this way, thedensity of the holograms recorded in media 208 or 208′ can be maderelatively high. And because it is only necessary to shift media 208 or208′ by the diameter of a beam waist, rather than the largest diameterof the signal beam 210 inside media 208 or 208′, media 208 or 208′ canadvantageously record a relatively greater number of holograms.

[0045] Further, because a method and apparatus in accordance with thepresent invention can accommodate holograms generated by the samereference beam and which can overlap at any point except that whichcoincides with the signal beam waist, the amount of divergence in asignal beam is less relevant. This provides at least two additionaladvantages. First, relatively higher numerical aperture lenses may beused to generate the signal beam without reducing the density ofholograms that can be recorded in media 208 by system 200. This isbecause the increased beam divergence of a higher numerical aperturelens has no effect on the geometrical limit to density of holograms thatcan be recorded in media 208. A second advantage is that relativelygreater thickness media can be used without decreasing the density ofthe holograms recorded in the media because, as discussed above,overlapping of any portion of a signal beam generating a hologram usingthe same reference beam does not effect readout of the hologram. Thus,overlapping of converging cones or diverging cones in a signal beam nearthe edges of relatively thick media is acceptable.

[0046] It is also considered that the method discussed above withrespect to system 200 can be used in systems using holographic opticalelements (HOE's) that function as lenses. HOE's are well known to thoseskilled in the art and disclosed, for example, in U.S. Pat. No.5,661,577 entitled “Incoherent/Coherent Double Angularly MultiplexedVolume Holographic Optical Elements”, which is incorporated in itsentirety by reference.

[0047]FIG. 5 is an alternate embodiment of a holographic system 300 inaccordance with the present invention for carrying out a method of thepresent invention. Holographic system 300 is set up to use what isreferred to as phase conjugate readout or reconstruction. Phaseconjugate readout is disclosed, for example, in “Optical PhaseConjugation” edited by Robert Fisher, Academic Press, 1993, ISBN0-12-257740-X. Phase conjugate readout is also disclosed in G. W. Burrand I. Leyva, “Multiplexed Phase-Conjugate Holographic Data Storage witha Buffer Hologram”, Optics Letters, 25(7), 499-501 (2000) which ishereby incorporated by reference. System 300 includes a reflective SLM,for encoding an incident signal beam with data to be stored in hologram,and a beam splitter 340 for directing an incident beam 350 into SLM 335and through FT lens 342 to generate a signal beam 310. Signal beam 310creates a hologram in holographic media 308 in a manner similar to thatdiscussed above with respect to holographic system 200. System 300 alsoincludes detector 346 which, as discussed below, is used duringreproduction of a hologram.

[0048] Unlike system 200, FT lens 342 is focused to generate a beamwaist 316 on the same side of media 308 as FT lens 342 rather than on anopposite side thereof. In this way, a diverging cone 314 of signal beam310, along with a reference beam 320 a, forms a hologram in media 308,as is understood in the art. System 300 also includes a filter block 330which, as discussed below, is used during readout of a hologram andwhich is placed in the same side of media 308 as FT lens 342. Likefilter block 230, filter block 330 includes an aperture 334 that islarge enough to allow the waist 316 of signal beam 310 to passthere-through. As discussed above with respect to system 200, aplurality of holograms can be multiplexed (e.g, angle, phase,wavelength) at the same location in media 108. Additionally, as alsodiscussed above, a plurality of holograms created using the samereference beam can be generated in holographic material 308 using signalbeams (not shown in FIG. 5) which diverging cones overlap and whichwaists do not overlap.

[0049]FIG. 6 illustrates the phase conjugate readout of a hologram insystem 300. To readout a hologram, a phase conjugate readout beam 321 ais directed into media 308. As used herein, phase conjugate readout beamindicates a readout beam that travels in a direction substantiallydiametrically opposite to that of the direction of a reference beam usedto create a hologram, but is otherwise substantially the same as thereference beam used to create the hologram. Thus, readout beam 321 atravels in substantially a diametrically opposite direction as referencebeam 320 a by is otherwise substantially the same as reference beam 320a. This generates an output beam 311 that has a first portion 315 awhich travels along a substantially opposite path from signal beam 310.As noted above, other holograms generated with the same reference beamas reference beam 320 a overlap with the hologram created by signal beam310. Thus, these other holograms will also be reproduced by reproductionbeam 320 b. One such reproduction is shown in FIG. 6 as included in asecond portion 315 b of output beam 311.

[0050] To filter output beam 311 such that only a portion 315 a ofoutput beam 311 that contains a reproduction of hologram created bysignal beam 310 reaches detector 346, filter block 330 is placed inoutput beam 311. In particular, the aperture 334 of filter block 330 isplaced at the waist 317 a of first portion 315 a of output beam 311 toallow the waist 317 a to pass through aperture 334. As noted above, thewaist of signal beams used to generate holograms overlapping with thehologram generated by signal beam 310 and reference beam 320 a are notoverlapped with the waist 316 of signal beam 310. Thus, when a secondportion 315 b of output beam 311 is also generated by reference beam 320b, filter block 330 is placed to filter out output beam second portion315 b at the waist 317 b thereof. In this way, information fromsubstantially only the hologram generated by signal beam 310 istransmitted through FT lens 342, into beam splitter 340 and ontodetector 346.

[0051] As discussed above, a method and apparatus in accordance with thepresent invention facilitates the used of relatively high numericalaperture lenses. However, relatively high numerical aperture lenses ofrelatively high quality (e.g. having relatively low aberration anddefects) can be relatively expensive to manufacture. However, use ofphase conjugate reproduction reduces the importance of using arelatively high quality lens. This is because aberrations and distortionplaced in a signal beam by a lens when a hologram is generated areremoved by the lens from the reconstructed object beam as it passes theopposite direction back through the lens to be detected. Thus, a phaseconjugate readout system, such as system 300, can advantageouslygenerate relatively high quality images in a relatively cost effectivemanner. Further, in addition to filtering out unwanted reconstructionsfilter block 330 filters the original signal beam 310 to band-limit thesignal before recording to reduce the size of the holograms, which isdesirable.

[0052] A method and apparatus in accordance with the present inventioncan also be implemented using an imaging system that does not use a lenson readout. Such systems are disclosed, for example, in Holographic DataStorage, edited by Hans Coufal, Springer-Verlag, 2000, pp. 29-30, whichis incorporated by reference. FIG. 7 is an illustration of analternative embodiment of an apparatus and method of the presentinvention which can implement a lens-less readout of a hologram. FIG. 7illustrates a holographic system 400 that includes a focusing lens 432for focusing an incident beam 450 through a transmission SLM 435 thatalso includes a detector which is used on reproduction of a hologram bysystem 400. Transmission SLMs that also include a detector are wellunderstood in the art.

[0053] Transmission SLM 435 encodes incident beam 450 with data to berecorded in a hologram in holographic media 408. Focusing lens 432focuses signal beam to form a beam waist 416 outside of media 408 and onthe same side of media 408 as SLM 435. A reference beam 420 a is presentin media 408 with a diverging cone 414 of signal beam 410 to create ahologram in media 408. System 400 also includes a filter block 430having an aperture 434 at the location of beam waist 416 to allow signalbeam 410 to pass through filter block 430 when recording a hologram. Asdiscussed above with respect to systems 200 and 300, a plurality ofholograms can be multiplexed at the same location in media 408 as signalbeam 410 generates a hologram. Additionally, as also discussed abovewith respect to systems 200 and 300, media 408 can be shifted by atleast an amount equal to the diameter of beam waist 416 of signal beam410 to generate additional holograms in media 408 which use the samereference beam as reference beam 420 a and which will overlap with ahologram created by signal beam 410.

[0054]FIG. 8 illustrates readout of a hologram in holographic system400. Like system 300 discussed above, system 400 uses a phase conjugatereadout beam 421 a to reconstruct a hologram from media 408. Readoutbeam 421 a will generate an output beam 411 which includes a firstportion 415 that carries substantially only a reproduction of thehologram recorded by signal beam 410. First portion 415 of output beam411 includes a waist 417 outside of media 408 at the aperture 434 offilter block 430. In this way, first portion 415 of output beam 411 canpass through block filter 430. As with systems 200 and 300 discussedabove, additional portions (not shown in FIG. 8) of output beam 411carrying information from additional holograms recorded in media 408using reference beam 420 a and overlapping with the hologram createdfrom signal beam 410 will also be generated by phase conjugate readoutbeam 421 a. However, because media 408 was shifted by at least an amountequal to the diameter of beam waist 416 of signal beam 410, filter block430 blocks these additional portions of output beam 411 from reachingthe detector of SLM 435.

[0055] Because no lens was used between SLM 435 and media 108 whengenerating a hologram in media 408, no lens is necessary whenreproducing a hologram from media 408. This can advantageously reducethe cost and size of a holographic system such as system 400.

[0056] Systems 300 and 400 discussed above each created holograms usingsignal beams having waists outside of the holographic media. Asdiscussed above with respect to FIG. 3a, however, it is also possible toimplement a polytopic multiplexing system in which holograms are createdusing signal beams having waists inside the holographic media. FIG. 9illustrates a holographic system 500 in accordance with the presentinvention and that generates holograms from signal beams that havewaists inside the holographic media. System 500 includes a beam splitter540 which directs an incident beam 550 into a reflective SLM 535 tocreate a signal beam 510. Signal beam 510 is then directed through afirst FT lens 542. A filter block 530 having an aperture 534 is placedin the path of signal beam 510 after first FT lens 542. During recordingfilter block 530 can bandpass limit the frequency of the signal beam 510thereby decreasing the size of a hologram generated by signal beam 510.FT lens 542 focuses signal beam 510 to a first waist 516 a at theaperture 534 of filter block 530. Preferably, aperture 534 is sized toallow signal beam 510 to pass there-through. After passing throughfilter block 530, signal beam 510 diverges again to pass through asecond FT lens 543 which straightens out signal beam again before itpasses through a third lens 544, which focuses signal beam 544 to asecond waist 516 b, and which may or may not be an FT lens.

[0057] As shown in FIG. 9, second waist of signal beam occurs insideholographic media 508 where signal beam interferes with reference beam520 a to generate a hologram. As discussed above with respect to systems200, 300 and 400, a plurality of holograms can be multiplexed at thesame location in media 508 as signal beam 510 generates a hologram.Additionally, as also discussed above with respect to systems 200, 300and 400, media 508 can be shifted by at least an amount equal to thediameter of second beam waist 516 b of signal beam 510 to generateadditional holograms in media 508 which use the same reference beam asreference beam 520 a and which will overlap with a hologram created bysignal beam 510.

[0058] When recording with a beam waist inside the media it is desirableto use a phase mask or phase element which shifts the phase of a beampassing therethrough to make the intensity in the material more uniform.This is well known in the art, and disclosed, for example, in“Holographic data storage”, edited by Hans Coufal, Spinger-Verlag,pp259-269 (2000) which is incorporated by reference. System 500 includesa phase element 547 between lenses 543 and 544, though phase element 547can be placed anywhere in signal beam 510 between beam splitter 540 andmedia 508, directly on the SLM, or possibly in the illumination beam 550right before beamsplitter 540. In order to make the beam more uniform inthe media phase element 547 also makes the beam waist 516 b larger. Thisdecreases the density achievable using any method including polytopic.If, however, phase element 547 is placed in system 500 after the filterblock 530, the size of aperture 534 does not have to be increased. Thisis advantageous in that it increases the achievable density.

[0059]FIG. 10 illustrates readout of a hologram in holographic system500. Like systems 300 and 400 discussed above, system 500 uses a phaseconjugate readout beam 521 a to regenerate a hologram from media 508.Readout beam 521 a will generate an output beam 511 which includes afirst portion 515 a that carries substantially only a readout of thehologram recorded by signal beam 510. As shown in FIG. 10, first portion515 a of output beam 511 travels back along substantially the same pathas signal beam 510, only in the opposite direction, through third FTlens 544 and second FT lens 543. Second FT lens 543 causes first portion515 a of output beam 511 to converge to a second beam waist 517 b.Because first portion 515 a of output beam 511 is traveling back alongsubstantially the same path as signal beam 510, second waist 516 b willoccur at, and pass through, aperture 534 of filter block 530. Firstportion 515 a of output beam 511 then passes through first FT lens 542,into beam splitter 540 and onto detector 546.

[0060] In system 500, it is also considered that the SLM and detector beco-located by using an integrated device with the detector elements inthe back plane of the SLM. Such SLMs integrated with a detector are wellunderstood in the art. This would result in a significant cost and sizereduction. It is also considered to integrate a phase element such asphase element 547 into the SLM 535 or SLM/detector.

[0061] If an integrated SLM/detector is not used in system 500,beamsplitter 550 can be a polarizing beamsplitter. In such a case, avariable waveplate 553 may be positioned between beamsplitter 550 andfirst FT lens 542, though variable waveplate 553 may be positionsanywhere in the path beamsplitter 550 and media 508. Other preferablelocations for variable waveplate 553 are after the block 530 or near theimage plane between lenses 543 and 542. On readout variable waveplate553 is changed so that the reconstructed signal beam is routed todetector 546 using beamsplitter 550 with minimal loss of intensity.

[0062] Because, as discussed above, additional holograms were generatedin media 508 using the same reference beam 520 a that generated thehologram created by signal beam 510. These additional holograms alsooverlap with the hologram created by signal beam 510. Thus, readout beam521 a will also reconstruct at least portions of these additionalholograms in output beam 511. As discussed above with respect to systems200, 300 and 400, readouts of these additional holograms are filteredout of the output beam. In particular, FIG. 10 shows second portion 515b of output beam 511 which represents a readout of one of the additionalholograms generated by the same reference beam 520 a that generated thehologram created by signal beam 510 and which overlaps with the hologramcreated by signal beam 510. Second portion 515 b of output beam 511passes out of media 508 and through third FT lens 544 and second FT lens543. Second FT lens 543 causes second portion 515 b or output beam 511to converge to a second beam waist 519 b. Because, as discussed above,media 508 was shifted by at least a distance equal to the diameter of abeam waist 516 b of signal beam 510, second beam waist 519 b of secondportion 515 b of output beam 511 will not overlap with second beam waist517 b of first portion 515 a of output beam 511. Thus, filter block 530is preferably positioned to block transmission of second portion 515 bof output beam 511 at second beam waist 519 b thereof. Therefore, onlyfirst portion 515 a of output beam 511 is transmitted to detector 546.

[0063] As discussed above, the combination of FT lenses 544 and 543 actto recreate a first beam waist 517 a, which is inside media 508, outsidemedia 508 at the location of second waist 517 b. As such, filtering ofoutput beam 511 to remove portions thereof having unwanted hologramreadouts can occur outside of media 508 even though second beam waist516 b of signal beam 510 occurs inside of media 508. Filter 534 also canband pass filter the object beam 510. In FIG. 11, the filtering of anobject beam and output beam is realized in another fashion. FIG. 11illustrates a system 600 in accordance with the present invention.System 600 includes a reflective SLM 635, beam splitter 640, firstangular filter 630 a and first lens 642. When recording a hologram inmedia 608, an incident beam is projected into beam splitter 640 toreceive data from SLM 635 and pass through angular filter 630 a and lens642 and into holographic media 608 to record a hologram therein with areference beam 620. In system 600, holographic media 608 is preferablylocated at the beam waist of the signal beam. In this way, multipleholograms can be angle, wavelength, correlation, peristrophic, fractalor otherwise multiplexed in media 608. As discussed above with respectto systems 200, 300, 400 and 500, media 608 can then be shifted by adistance at least a wide as the waist of a signal beam recording ahologram. The same reference beam can then be used to record additionalstacks of holograms in media 608. The angle filters 630 a and 630 b areshow on the SLM and detector sides of the lenses, but it is alsopossible for them to be on the media side of the lens.

[0064]FIG. 11 illustrates the readout of a hologram from media 608. Areadout beam 620 is used to generate an output beam 611 that includesportions having readouts of multiple holograms that were each generatedusing a reference beam that was the same as readout beam 620 and thatwere overlapped in media 608. A first portion 615 of output beam 611includes a readout of one such hologram. Output beam 611 passes throughsecond lens 644, second angular filter 630 b, and onto detector 646.

[0065] Angular filters 630 a and 630 b achieve the same filtering as anaperture filter block, such as filter block 230 discussed above, in theFourier transform domain. Angular filters 630 a and 630 b operate tolimit the angular bandwidth of the signal pixels in a Fresnel plane.That is, angular filters 630 a and 630 b filter out optical rays of thesignal that are traveling at a certain cutoff angle relative to theoptical axis of the system. Angular filters 630 a and 630 b can havedifferent pass bands which are equivalent to different sized holes inthe Fourier Plane. Angular filters 630 a and 630 b allow for thefiltering of the signal beam to occur at intermediate planes in thesystem rather than near or at a Fourier plane or image plane. Angularfilters 630 a and 630 b can be made by a multilayer thin film coating,similar to coating for notch filters and reflective coatings as is wellknown in the art, volume holographic gratings or a holographic opticalelement (“HOE”), or a combination of both.

[0066] Angular filter 630 a is used before media 608 to limit thebandwidth of the signal beam such that holograms recorded in media 608can be smaller. Angular filter 630 b is used on readout to filter outunwanted hologram readouts from output beam 611. As shown in the exampleof FIG. 11, angular filter 630 b allows substantially only first portion615 of output beam 611 to pass to detector 646. It is also consideredthat a system and method in accordance with the present invention useangular filtering in reflection and phase conjugate geometries as well,both of which are well understood in the art. For example, withoutlimitation, filter block 330 of system 300 discussed above could bereplaced with an angular filter. It is also within the ambit of thepresent invention to use a combination of aperture filter blocks andangular filters. For example, an angular filter could be used in system200, shown in FIG. 3, between SLM 240 and FT lens 242 to limit thebandwidth of the object beam of system 200. It is also considered thatthe filter blocks of systems 300, 400 and 500 be replaced with angularfilters.

[0067] Many uses of holograms require making copies of a holographic“master” medium. The Handbook of Optical Holography, Academic Press1979, pp. 373-377, incorporated by reference, describes replication ofnon multiplexed holograms. One method, “copying by reconstruction”,first reconstructs the image and thereafter records a new hologram,using the reconstructed image. This method is applicable to thick aswell as thin holograms. It has been used for multiplexed holograms byreconstructing and copying individual holograms one at a time. Inaddition, a whole “layer” of the media can be copied in at the sametime—that is, all the holograms in the media multiplexed at a certainangle can be copied by reconstruction at the same time.

[0068] Copying by reconstruction can be included in a method anapparatus of the present invention. The multiplexing step still allowsindividual hologram reproduction or whole “layer” replication. Layerreplication can be used for stacks of holograms that overlap withpolytopic multiplexing. Layer replication of polytopic multiplexedholograms in accordance with the present invention is illustrated inFIG. 12. FIG. 12 illustrates prerecorded media 810 includes a pluralitypolytopic multiplexed stacks of holograms 814. Multiplexing of holograms814 in individual stacks can be by angle, wavelength, phase or any othermethod. Blank media 812 is placed beneath prerecorded media 810. Areadout beam 816 illuminates the plurality of multiplexed stacks ofholograms 814 such that one hologram from each stack will be readout andcopied into blank media 812. Readout beam 816 can then be altered (bywavelength, angle, phase, etc.) to cause a second hologram from eachstack to be readout and copied into black media 812. In this way, eachof the plurality of holograms 814 in prerecorded media 810 can be copiedinto blank media 812.

[0069] Alternatively multiple holograms or stacks of holograms can besimultaneously reconstructed and copied by use of multiple,mutually-incoherent reference beams. The reference beams would havedifferent angles or wavelengths, etc depending on how the holograms weremultiplexed in the stack. This would allow whole stacks to be copied atonce. This procedure is analogous to the one described in OpticsLetters, vol. 17, no 9, pp 676-678, which is incorporated by referencein its entirety. In using polytopic multiplexing the adjacentoverlapping stacks or a “layers” in these stacks could be replicatedwith mutual-incoherent beams. This would significantly reduce cross talknoise during the replication process. Thus instead of one beam toreplicate the entire “layer” of holograms, multiple mutually-incoherentbeams could be used.

[0070]FIGS. 3 and 3a show examples of systems in accordance with thepresent invention in which the beam waist of the object beams is in theFourier transform plane. And, FIG. 11 illustrates an example of a systemin accordance with the present invention in which the beam waist is inthe imaging plane. A system and method of the present invention can alsobe realized with a combination of Fourier Transform in one dimension andimage plane in the other dimension. Such systems, not implementingpolytopic multiplexing, are disclosed, for example, in U.S. Pat. No.5,339,305 to Curtis et al. entitled “Disk-Based Optical Correlator andMethod” and K. Curtis and D. Psaltis, “Three-dimensional Disk BasedOptical Correlator”, Optical Engineering, Vol. 33, No. 12, December1994, both of which are incorporated by reference in their entirety.

[0071] An example of such a system is shown in FIG. 13. FIG. 13illustrates an optical system 700. System 700 includes an SLM 735through which an incident beam 750 is encoded with information to becomesignal beam 710. Though SLM 735 is a transmission SLM, it is within theambit of the invention to also use a reflection SLM. Additionally, SLM735 can include either a one dimensional array or pixels or a twodimensional array of pixels. Before reaching holographic media 708,signal beam 710 passes through a first 4F image system formed bycylindrical lenses 741 a and 741 b. The cylindrical lenses 741 a and 741b are each oriented in an “x” direction such that the cylindrical axesof lenses 741 a and 741 b are parallel. Between cylindrical lenses 741 aand 741 b is cylindrical FT lens 742 a oriented in a “y” directionperpendicular to the x-direction and in a direction perpendicular to theplane of FIG. 12. Both the x-direction and y-direction are perpendicularto a direction of propagation of object beam 710. Cylindrical lenses 741a and 741 b image object beam 710 into circular media 708 in thex-direction and cylindrical FT lens 742 a Fourier transforms object beam710 in the y-direction to generate holograms in circular media 708 withreference beam 720. Media 708 is preferably in the form of a disk thatrotates about a central axis but can also be in the shape of a card withthe holograms stored along lines. Thus, holograms are overlapped incircular formation when media 708 is shifted.

[0072] As discussed above, a plurality of holograms can be multiplexedat the same location in media 708 using different reference beams tocreate a stack of holograms. Additionally, media 708 can be rotated toallow creation of additional stacks 752 of holograms using the samereference beams and that can overlap with each other. To readoutholograms from stacks 752, a readout beam is used to transmit an outputbeam 711 through a filter block 730. In the example of system 700,holograms are preferably generated in media 708 such that both the imagewaist and Fourier transform waist of object beam 710 are located outsideof media 708 at the location of filter block 730. To achieve this, thefocal length of cylindrical FT lens 742 is twice the focal length oflenses 741 a and 741 b.

[0073] Additionally, stacks 752 of holograms are preferably overlappedin media 708 such that neither the Fourier Transform waist nor the imagewaist of the object beams generating the stacks 752 of hologramsoverlap. Thus, an aperture 730 a of filter block 730 can be sized tofilter out readouts of unwanted overlapped holograms from output beam711. To accomplish this, the dimension of an aperture 730 a in anx-direction is preferably the size of the image beam waist. And, thesize of the slit in the y-direction is preferably the Nyquist aperturediscussed above.

[0074] After passing through filter block 730, output beam 711 passedthrough three additional cylindrical lenses 743 a, 743 b and cylindricalFT lens 742 b before reaching detector 746. Cylindrical lenses 743 a and743 b form a second 4F system extended in the x-direction such that thecylinder axes of lenses 743 a and 743 b are parallel in the x-direction.Cylindrical FT lens 742 b is positioned between lenses 742 b and 743 band has a cylinder axis that is parallel to the y-direction. In thisway, a hologram presented on SLM 735 and stored in the media is thenreconstruct as an image focused on detector 746.

[0075] It is also considered that a system implementing polytopicmultiplexing and having a Fourier transform in a direction perpendicularto an image direction, such as system 700, use an angular filter, suchas angular filters 630 a and 630 b of system 600 above, and/or includemore than one filter. It is also contemplated that the Fourier transformplane of object beam 710 and output beam 711 does not spatially coincidewith the image plane of object beam 710 and output beam 711,respectively. In such a case, filtration for the Fourier transform planecould occur at a different location that filtration for the image plane.Also, the filters for the image plane and Fourier transform plane wouldbe slits that extend in the direction of the respective cylindrical lenssystem.

[0076] It is also considered that a system implementing polytopicmultiplexing and having a Fourier transform in a direction perpendicularto an image direction, such as system 700 be implemented in a phaseconjugate architecture, such as system 300. Such a system could alsoplace either one or both of the image beam waist and Fourier transformwaist inside the holographic media and include a lens system to relaythe respective waist outside of the media where it is filtered, such asin system 500 shown in FIGS. 9 and 10.

[0077] Various modifications to the preferred embodiments can be madewithout departing from the spirit and scope of the invention. Forexample, and without limitation, different optical arrangements andrecording geometries such as reflective or transmissive geometries arecontemplated. The holograms can be recorded in the media at an imageplane, Fourier plane or at any intermediate plane. The media could be inthe form of a disk, card, tape or any other form. The relative motionfor moving the beam waist for the next stack of multiplexed hologramscould be achieved by moving the media, moving the optical system, acombination of both moving media and optics or by beam steering thebeams to the new location. This relative motion between stacks could besubstantially linear motion, substantially a rotation, or some othertrajectory. Thus the overlapping stacks could form lines, circles,ellipses, or spirals for example. Devices that are read only devices(ROMs), record only apparatus, as well as devices that record and readout are also considered. Thus, the foregoing description is not intendedto limit the invention which is described in the appended claims.

What is claimed is:
 1. A method enabling holographic recording andreadout including: creating a first hologram in a holographic mediausing a first reference beam and a first signal beam, the first signalbeam having a beam waist; creating a second hologram using a secondreference beam that is the same as the first reference beam and a secondsignal beam, the second signal beam having a beam waist; overlapping atleast a portion of the second hologram with the first hologram; andseparating the first hologram from the second hologram such that noportion of the beam waist of the first signal beam occurs at the samelocation as any portion of beam waist of the second signal beam.
 2. Themethod of claim 1 including: reconstructing the first hologram in afirst portion of an output beam; reconstructing at least the secondhologram in a second portion of the output beam; and filtering theoutput beam to substantially contain only a reconstruction of the firsthologram.
 3. The method of claim 2 wherein filtering the output beamincludes placing a filter block in the output beam, the filter blockhaving an aperture which allows information from the first hologram topass through the filter block.
 4. The method of claim 3 wherein: thefirst portion of the output beam has a first output waist and the secondportion of the output beam has a second output waist; and filtering theoutput beam includes: placing the aperture of the filter block at alocation of the first output waist; and blocking transmission of thesecond portion of the output beam at the second output waist.
 5. Themethod of claim 4 wherein one dimension of the aperture is the Nyquistsize.
 6. The method of claim 4 wherein one dimension of the aperture istwice the Nyquist size.
 7. The method of claim 2 wherein creating thefirst hologram includes: placing a spatial light modulator (SLM) in thepath of an incident beam to generate the first signal beam; and placinga lens in the path of the first signal beam between the SLM and theholographic media.
 8. The method of claim 7 wherein regenerating thefirst hologram includes using a readout beam to create the output beamwherein the readout beam is the same as the first reference beam.
 9. Themethod of claim 8 wherein regenerating the first hologram includes usinga readout beam to create the output beam, wherein the readout beam isthe phase conjugate of the first reference beam.
 10. The method of claim9 including detecting a readout of the first portion of the output beamin a detector that is integrated with the SLM.
 11. The method of claim 2wherein creating the first hologram includes: placing a spatial lightmodulator (SLM) in the path of an incident beam to generate the firstsignal beam; and placing a lens in the path of the incident beam beforethe incident beam reaches the SLM; and transmitting the signal beam fromthe SLM to the holographic media without passing the signal beam througha lens.
 12. The method of claim 11 wherein reconstructing the firsthologram includes using a readout beam to create the output beam,wherein the readout beam is the phase conjugate of the first referencebeam.
 13. The method of claim 12 including detecting a first portion ofthe output beam in a detector that is integrated with the SLM.
 14. Themethod of claim 2 wherein filtering the output beam includes placing anangular filter in the output beam.
 15. The method of claim 14 whereinthe angular filter is a layered film.
 16. The method of claim 14 whereinthe angular filter is a holographic optical element. HOE.
 17. The methodof claim 14 wherein reconstructing the first hologram includes using areadout beam to create the output beam, wherein the readout beam is thephase conjugate of the first reference beam.
 18. The method of claim 2including filtering the first signal beam before forming a hologram tolimit the bandwidth of the first signal beam.
 19. The method of claim 18wherein: filtering the first signal beam includes filtering the signalbeam with one of either an angular filter and a filter block; andfiltering the output beam includes filtering the output beam with one ofeither an angular filter and a filter block.
 20. The method of 2wherein: creating the first hologram includes placing a first waist ofthe first signal beam inside the holographic media; and creating thesecond hologram includes placing a first waist of the second signal beaminside the holographic media.
 21. The method of claim 20 including:generating a second waist of the first signal beam outside theholographic media; generating a second waist of the output beam outsidethe holographic media; and blocking the second portion of the outputbeam outside the holographic media at the second waist of the outputbeam.
 22. The method of claim 21 wherein regenerating the first hologramincludes using a readout beam to create the output beam wherein thereadout beam is the same as the first reference beam.
 23. The method ofclaim 21 wherein regenerating the first hologram includes using areadout beam to create the output beam, wherein the readout beam is thephase conjugate of the first reference beam.
 24. The method of claim 23including: detecting the first portion of the output beam in a detector;and passing the first portion of the output beam through a waveplate anda polarized beam splitter before detecting the first portion of theoutput beam.
 25. The method of claim 20 including passing the firstobject beam through a phase element prior to reaching the holographicmedia.
 26. The method of claim 2 wherein filtering the output beamincludes forming a filter that is integrated with the holographic media,the filter having at least one aperture.
 27. The method of claim 2including: locating the holographic media at an image plane of the firstsignal beam such that the beam waist of the first signal beam isprojected into the holographic media; projecting the first signal beamthrough a first angular filter before projecting the first signal beaminto the holographic media; and wherein filtering the signal beamincludes projecting the output beam through a second angular filter. 28.The method of claim 27 including generating the output beam using areadout beam that is the same as the first reference beam.
 29. Themethod of claim 1 including: multiplexing a first plurality of hologramswith the first hologram at a first multiplex location in the holographicmedia; multiplexing a second plurality of holograms with the secondhologram at a second multiplex location in the holographic media. 30.The method of claim 29 wherein the first plurality of holograms areangle multiplexed at the first multiplex location and the secondplurality of holograms are angle multiplexed at the second multiplexlocation.
 31. The method of claim 29 wherein the first plurality ofholograms are wavelength multiplexed at the first multiplex location andthe second plurality of holograms are wavelength multiplexed at thesecond multiplex location.
 32. The method of 1 wherein: creating thefirst hologram includes placing the beam waist of the first signal beamoutside the holographic media; and creating the second hologram includesplacing the beam waist of the second signal beam outside the holographicmedia.
 33. The method of claim 1 including locating the holographicmedia at an image plane of the first signal beam.
 34. The method ofclaim 1 locating the holographic media at a Fourier plane of the firstsignal beam.
 35. A method for reading out a first hologram created in aholographic media from a first signal beam and a first reference beam,the first hologram overlapping in the holographic media with a portionof at least a second hologram created by a second signal beam and areference beam that is the same as the first reference beam such that noportion of a beam waist of the first signal beam occurs at the samelocation as any portion of a beam waist of the second signal beam,including: reconstructing the first hologram in a first portion of anoutput beam and reconstructing at least the second hologram in a secondportion of the output beam; and filtering the output beam tosubstantially contain only a reconstruction of the first hologram. 36.The method of claim 35 including locating the holographic media at animage plane of the first signal beam.
 37. The method of claim 35locating the holographic media at a Fourier plane of the first signalbeam.
 38. The method of claim 2 including: imaging the first signal beamon a first spot in the holographic medium in a first direction; andFourier transforming the first signal beam on the first spot in theholographic medium in a direction orthogonal to the first direction. 39.An apparatus for enabling recordation and readout a hologram including:a first signal beam and a first reference beam generating a firsthologram in a holographic medium the first signal beam having a firstbeam waist; a second signal beam and a second reference beam, the secondreference beam the same as the first reference beam, the second signalbeam and the second reference beam generating a second hologram in aholographic medium the second signal beam having a second beam waist, atleast a portion of the first hologram spatially overlapping with atleast a portion of the second hologram in the holographic media suchthat no portion of the waist of the first signal beam occurs in the samelocation in the holographic media as any portion of the waist of thesecond signal beam.
 40. The apparatus of claim 39 including: an outputbeam having: a first portion that includes a readout of the firsthologram; and a second portion that includes a readout of the secondhologram reconstructs; and a filter for filtering at least the secondportion out of the output beam.
 41. The apparatus of claim 40 whereinthe filter includes an opaque filter block having an aperture.
 42. Theapparatus of claim 41 wherein: the first portion of the output beam hasa first output waist and the second portion of the output beam has asecond output waist; and in reproducing the first hologram the aperturein the filter block is located at the first output waist andtransmission of the second portion of the output beam is blocked at thesecond output waist.
 43. The apparatus of claim 42 wherein one dimensionof the aperture in the Nyquist size.
 44. The apparatus of claim 42wherein one dimension of the aperture is twice the Nyquist size.
 45. Theapparatus of claim 40 further including: an incident beam; a spatiallight modulator (SLM) located in a path of the incident beam to generatethe first signal beam; and a lens located in the path of the firstsignal beam between the SLM and the holographic media.
 46. The apparatusof claim 45 further including a readout beam that generates the outputbeam wherein the readout beam is the same as the first reference beam.47. The apparatus of claim 45 further including a readout beam thatgenerates the output beam wherein the readout beam is the phaseconjugate of the first reference beam.
 48. The apparatus of claim 47including a detector for detecting the output beam wherein the detectoris integrated with the SLM.
 49. The apparatus of claim 40 including: anincident beam; a spatial light modulator (SLM) located in the path of anincident beam to generate the first signal beam; a lens located in thepath of the incident beam before the incident beam reaches the SLM suchthat the signal beam is transmitted directly from the SLM to theholographic media without passing through any lens.
 50. The apparatus ofclaim 49 including a readout beam to create the output beam, wherein thereadout beam is the phase conjugate of the first reference beam.
 51. Theapparatus of claim 50 including a detector for detecting the output beamwherein the detector is integrated with the SLM.
 52. The apparatus ofclaim 40 wherein the filter includes an angular filter.
 53. Theapparatus of claim 52 wherein the angular filter is formed from layersof film.
 54. The apparatus of claim 52 wherein the angular filter isformed from an HOE.
 55. The apparatus of claim 52 including a readoutbeam to generate the output beam wherein the readout beam is the phaseconjugate of the first reference beam.
 56. The apparatus of claim 40including a second filter placed in the first object beam to limit thebandwidth of the first object beam.
 57. The apparatus of claim 56wherein: the first filter includes one of either an angular filter and afilter block having an aperture; and the first filter includes one ofeither an angular filter and a filter block having an aperture.
 58. Theapparatus of claim 40 wherein in forming the first hologram the firstbeam waist is located inside the holographic media and in forming thesecond hologram the second beam waist is located inside the holographicmedia.
 59. The apparatus of claim 58 including: a second beam waist ofthe first signal beam, the second beam waist of the first signal beamlocated outside of the holographic media; a second beam waist of theoutput beam, the second beam waist of the output beam located outsidethe holographic media wherein the second portion of the output beam isblocked at the second beam waist of the output beam.
 60. The apparatusof claim 59 including a readout beam used to generate the output beam,wherein the readout beam is the same as the first reference beam. 61.The apparatus of claim 59 including a readout beam used to generate theoutput beam, wherein the readout beam is the phase conjugate of thefirst reference beam.
 62. The apparatus of claim 61 including: adetector for detecting the output beam; a polarized beam splitterlocated in a path of the output beam before the detector; and awaveplate located in the path of the output beam before the polarizedbeam splitter.
 63. The apparatus of claim 58 including a phase elementlocated in a path of the first signal beam before the holographic media.64. The apparatus of claim 40 wherein the filter is integrated with theholographic media.
 65. The apparatus of claim 40 wherein the firstsignal beam includes an image plane that is projected into theholographic media.
 66. The apparatus of claim 65 including a firstangular filter located in a path of the first signal beam in front ofthe holographic media; and the filter includes a second angular filterlocated in a path of the output beam.
 67. The apparatus of claim 66including a readout beam that is used to generated the output beamwherein the readout beam is the same as the first reference beam. 68.The apparatus of claim 39 including: a first plurality of hologramsmultiplexed with the first hologram at a location of the first hologramin the holographic media; and a second plurality of hologramsmultiplexed with the second hologram at a location of the secondhologram in the holographic media.
 69. The method of claim 68 whereinthe first plurality of holograms are angle multiplexed at the firstmultiplex location and the second plurality of holograms are anglemultiplexed at the second multiplex location.
 70. The method of claim 68wherein the first plurality of holograms are wavelength multiplexed atthe first multiplex location and the second plurality of holograms arewavelength multiplexed at the second multiplex location.
 71. Theapparatus of claim 39 wherein in forming the first hologram the firstwaist is located outside the holographic media and in forming the secondhologram the second waist is located outside the holographic media. 72.The apparatus of claim 39 wherein an image plane of the first signalbeam is projected inside the holographic media.
 73. The apparatus ofclaim 39 wherein a Fourier plane of the first signal beam is projectedinside the holographic media.
 74. An apparatus for reading out a firsthologram created in a holographic media from a first signal beam and areference beam, the first hologram overlapping in the holographic mediawith a portion of at least a second hologram created by a second signalbeam and the reference beam such that no portion of a beam waist of thefirst signal beam occurs at the same location as any portion of a beamwaist of the second signal beam, including: an output beam having: afirst portion in which the first hologram is read out; and at least asecond portion in which the second hologram is read out; and a filter inthe output beam that filters the output beam to contain substantiallyonly a reconstruction of the first hologram.
 75. The apparatus of claim74 including locating the holographic media at an image plane of thefirst signal beam.
 76. The apparatus of claim 74 locating theholographic media at a Fourier plane of the first signal beam.
 77. Theapparatus of claim 39 wherein the first signal beam is imaged in a firstdirection on a spot in the holographic media and Fourier transformed onthe spot in the holographic media in a second direction orthogonal tothe first direction.
 78. A holographic media including: a firstplurality of holograms multiplexed with a first hologram at a firstlocation in the holographic media, the first hologram generated by afirst signal beam and a first reference beam, the first signal beamhaving a first beam waist; and a second plurality of hologramsmultiplexed with a second hologram at a second location in theholographic media, the second hologram created by a second signal beamand a second reference beam that is the same as the first reference beamthe second signal beam having a second beam waist, wherein a portion ofthe first hologram overlaps with a portion of the second hologram andthe first beam waist does not occur at the same location as the secondbeam waist.
 79. The holographic media of claim 78 wherein theholographic media is in the form of a disk and the first plurality ofholograms and the second plurality of holograms are overlapped in acircular formation.
 80. The holographic media of claim 78 wherein thefirst plurality of holograms and the second plurality of holograms areoverlapped in a line.
 81. The holographic media of claim 80 wherein theholographic media is in the form of a card.
 82. The holographic media ofclaim 80 wherein the holographic media is in the form of a tape.
 83. Amethod of replicating multiplexed holograms in the holographic media ofclaim 78 including: placing a blank holographic media adjacent to theholographic media of claim 78; projecting a readout beam through theholographic media of claim 78 to generate an output beam, the readoutbeam being the same as the first reference beam; and recording at leastthe first and second holograms into the blank holographic media usingthe output beam.